The typical electric arc spray metalizing apparatus utilizes a spray gun in which a pair of metal wires are brought together at an intersection point. Each of the metal wires is separately charged with electrical current. At the intersection point of the two wires, an electric arc is created. The electric arc is of sufficient energy to melt the wires. A jet or stream of compressed gas, usually air, is focused on the intersection point. The air atomizes the molten metal into particles and propels them in a spray stream onto a substrate. The separately charged wires are continuously driven forward and the electrical arc is maintained at the intersection point as the ends of the wires are continuously consumed in the energy of the arc. A coating is formed on the substrate as the metalized particles impact and distribute over the substrate.
According to the predetermined type of wire used, coatings having specific or predetermined surface characteristics can be found. For example, coatings having high resistance to corrosion may be achieved, while other coatings may achieve predetermined characteristics for oil retention, surface porosity, wear resistance, surface hardening and the like. By applying repeated coatings, worn parts can be rebuilt by increasing their thickness. Parts rejected in a manufacturing process due to mis-machining during production can be salvaged by building up the surface through multiple coatings and then machining the part to the correct dimensions. Castings can be saved by applying a metalized coating to the surface of the casting to decrease unacceptable porosity. In general, the advantages of electric arc metalizing, compared to other thermal spraying techniques, include faster deposition rates, greater bond strengths, less elaborate surface preparation, and reduced oxides.
One of the most common uses for spray metalizing technology is for coating equipment and structures to inhibit corrosion. The structure, equipment or other substrate is typically sandblasted prior to applying the metalizing coating. The sandblasting removes old coating materials and corrosion from the substrate. The sandblasting also creates substantial dust, grit, and other airborne particles in the environmental area where the spray metalizing apparatus is in use. An over-spray effect from the spray stream of molten metal particles is also present, due to the spraying aspects of metalizing technique. The dust, grit and other airborne particles have created particular problems in achieving the desired use of some types of prior electric arc spray metalizing equipment.
The dust, grit and airborne particles have a tendency to be attracted to the charged wires and to be drawn into the hollow cables through which the wires are directed to the spray gun. An accumulation of the dust, grit and other particles within the cables can increase the movement resistance of the wires and may ultimately prevent the wires from being reliably fed through the cables. Added or variable movement resistance can also cause uneven drive rates of one wire with respect to the other. The electric arc may even extinguish if radical differences exist between the drive rates. The position of the intersection point may be altered, thus altering the spray stream pattern of molten metal particles. An altered spray stream pattern may result in an uneven coating as a result of an uneven deposition of molten particles on the substrate. If ends of the intersecting wires are not approximately equally consumed in the arc, relatively large chunks or pieces of unmelted wire material may be transferred in the spray stream onto the substrate, thereby creating undesirable surface characteristics in the coating. Of course, aggravated wire feeding problems make the selective starting and stopping of the electric arc more difficult, thus increasing the overall time and cost required to coat a given area of substrate surface.
The potential for wire feeding problems also increase when relatively long cables are employed. Relatively long cables are sometimes desired to allow the spray gun to be more easily manipulated or when the size of the structure, equipment or substrate requires greater movement to cover the surface. Longer cables also increase the movement resistance of the wires.
For reasons of economy, it is usually desirable to coat relatively large surface areas rapidly. To achieve a high capacity metalizing coating effect, larger currents must be supplied, the wires must be driven faster or at greater rates, and wires of increased diameter must be employed. The larger wires provide more metal, and greater electrical currents assure that sufficient electrical power is available to consume the larger wires in the electric arc.
A variety of different wire feed mechanisms have been employed in electric arc spray metalizing apparatus. One type employs drive wheels positioned in the spray gun to pull the wires through the cables to the spray gun. Generally speaking, these drive wheels are relatively small and lack sufficient capability to grip the wires to pull them through relatively long cables in high capacity situations. Incorporating the drive wheels in the spray gun also increases the weight of the spray gun, making manipulation of the spray gun more difficult and tiresome. Another type of wire feed mechanism is a pushing unit, typically positioned at the end of the cable spaced away from the spray gun. While pushing wire feed mechanisms may develop greater force, many times they too are insufficient to adequately and reliably feed large wire through relatively long cables in high capacity situations. A third type of wire feed mechanism employs both a pulling drive mechanism positioned in the spray gun and a pushing drive mechanism positioned at the remote end of the cables. This type of arrangement usually incorporates the disadvantages of both the pushing and pulling mechanisms, without solving the problems of either type of mechanism, but increasing the complexity of the wire feed arrangement.
Unsatisfactory performance has been particularly perplexing in high capacity electric arc spray metalizing situations. Attempting to increase the spray metalizing capacity tends to magnify the effects of many of the disadvantages described above, and as a result, few, if any, high capacity electric arc spray metalizing devices are believed to exist. No known prior manually-operable electric arc spray metalizing device has satisfactorily solved all of the above described problems in a single piece of equipment or apparatus, under conditions of reliable and convenient use.